Re: "... the window start at a meaningless point within a line."

[Eli Zaretskii]

> Date: Sat, 17 Oct 2015 16:31:05 +0300
> From: Eli Zaretskii <address@hidden>
> Cc: address@hidden
> 
> Is the following a correct definition of when Fvertical_motion
> overshoots, i.e. lands one line below (for LINES > 0) the correct one:
> when the offset between the "actual" window-start and the preceding
> "xdisp" BOL is greater than the X coordinate of the last character of
> the 1st physical line in the window.

Another idea: did you try augmenting the X coordinate of the iterator
after it gets back to point at the beginning of Fvertical_motion?
IOW, after this line:

        move_it_to (&it,
                    (!disp_string_at_start_p
                     || FETCH_BYTE (IT_BYTEPOS (it)) == '\n')
                    ? PT
                    : PT - 1,
                    -1, -1, -1, MOVE_TO_POS);

modify it.current_x such that it's relative to the "actual"
window-start.  Then let Fvertical_motion do its job as usual.  Did you
try that?